Techniques relating to the center of gravity of a writing implement have been proposed. For example, a writing implement is disclosed in JP 2001-270281 A (Patent document 1) having its center of gravity in a middle region of a barrel having front, middle and back regions and capable of being stably handled in minute, reciprocating, turning motions for writing.
The inventors of the present invention conducted studies to develop a writing implement that can be easily handled in minute, reciprocating, turning motions and found that a writing implement makes minute, reciprocating, turning motions to move its writing point up and down as shown in FIG. 3 when the writing point is shifted from the end of the last stroke of one letter to a position to start writing the next letter. A writing implement capable of being easily handled in minute, reciprocating, turning motions has an axis of turning passing corresponding to or substantially corresponding to its center of gravity. It is inferred that such a writing implement can be easily handled in reciprocating, turning motions because the writing implement has a small rotational inertia about the axis of turning.
The center of gravity of the writing implement disclosed in JP 2001-270281 A including the barrel having the front, the middle and the back region can be set in the middle region by taking into consideration the total weight of the writing implement and the respective weights of the component parts. However, the writing implement constructed simply so that the center of gravity is in the middle region of the barrel could not give a hand gripping the writing implement satisfactory sensation of balance and stability.
A writing implement designed to place the center of gravity in its middle region, ignoring weight distribution is not necessarily capable of being satisfactorily handled in minute, reciprocating, turning motions.
A region denoted by the term “middle region” in Patent document 1 extends from a range having a diameter decreasing in a curve from a front end toward a middle part to a range having a diameter decreasing in a curve from the back end toward the middle part. According to the definition of the term “middle region”, the middle region could be a part near the tip of the writing implement or near the push button, depending on the shape of the writing implement. Thus, the position of the middle region defined by this term is very vague and hence the conception of weight distribution cannot be introduced into the writing implement mentioned in Patent document 1.
If weight distribution is ignored and an attention is paid only to the position of the center of gravity, the center of gravity can be set in the middle region even if front and back parts of the writing implement are formed from metallic parts. However, such a writing implement is ill-balanced and is unable to give a satisfactory sensation of stability to the hand gripping the writing implement. When the writing implement is gripped by a hand in a tilted position for writing, the metallic parts respectively having high specific weights destroy the balance and spoil the stability of the writing implement. When the front and the back part remote from the center of gravity are formed from metallic parts respectively having high specific weights, the writing implement has a large rotational inertia and cannot be smoothly operated in minute, reciprocating, turning motions.
As generally known, rotational inertia indicates the degree of difficulty in starting a body for rotation and the degree of difficulty in stopping a rotating body. Generally, it is easier to turn a straight bar about an axis passing through the center of the straight bar than turning the same straight bar about an axis passing through one of its ends because the diameter of a circle followed by an end of the straight bar when the straight bar is turned about the axis passing through the center of the straight bar is smaller than that of a circle followed by one of the ends of the straight bar when the straight bar is turned about the axis passing through the other end of the straight bar. Thus, the position of the center of gravity of a body is closely related with the rotational inertia of the body; the rotational inertia of the body is the least if the axis of rotation passes through the center of gravity of the body and the rotational inertia increases with the distance of the axis of rotation from the center of gravity.
Therefore, the rotational inertia can be reduced by making the axis of rotation pass through the center of gravity. However, it was found that the facility of handling the writing implement in minute, reciprocating, turning motions cannot be improved simply by setting the axis of turning of the writing implement at the center of gravity because the writing implement consists of many component parts and the rotational inertia of the writing implement is the sum of the respective rotational inertias of the component parts.
The least rotational inertia of a writing implement can be expressed by:I=dI1+dI2+dI3+ . . . +dIn where I is the minimum rotational inertia of the writing implement, and dI1, dI2, dI3, . . . are the rotational inertias of the component parts A, B, C, etc. about an axis of rotation passing through the center of gravity of the writing implement.